The present invention relates generally to the art of photoconductive devices such as solar cells for converting light into electrical energy and, more particularly, to a photoconductive device containing an electroless metal deposited conductive layer.
Photoconductive devices generally consist of a photoconductor capable of generating an electrical potential upon being exposed to light and contacts which are effective to draw off any electric current which results from irradiation of the photoconductor. In most instances, such photoconductive devices also contain a suitable substrate such as glass, in order to provide protection from the environment and to serve as a base for the photoconductor.
Many different photoconductive materials are known, such as silicon, germanium, gallium arsenide, and copper indium diselenide, to name but a few. Photoconductors comprising silicon have obtained particularly wide usage because of their economy. Originally, single crystal silicon photoconductors were widely used. However, recently, thin film alloys of silicon and hydrogen (TFS) have come to be preferred because of their lower cost and ease of fabrication. In particular, TFS layers incorporating a P-layer, an I-layer, and an N-layer, known as P-I-N photoconductors, have been utilized. The I-layer, the intrinsic layer, is generally formed from a thin layer of microcrystalline or amorphous silicon alloyed with hydrogen or a silicon hydride. The P-layer is formed from silicon doped with boron or similar dopant. Positive charge carriers, known as holes, are predominant in the P-layer. The N-layer is formed from silicon doped with phosphorous or similar dopant. Electrons are predominant in the N-layer.
Contacts on the P-layer and the N-layer permit electric current to be drawn off into an external circuit for use therein. More recently, such contacts have taken the form of thin films, such as films of tin oxide, indium tin oxide, or other transparent conductive oxide. The use of zinc oxide films, for example, is disclosed in copending application of Steven C. Lewis et al., Ser. No. 740,945, filed June 4, 1985, now U.S. Pat. No. 4,623,601, assigned to the same assignee as the present invention. Such conductive oxides are relatively transparent in reasonable thicknesses. They have been used primarily as the front contact in solar cells, that is, the contact adjacent to the transparent substrate, which may face or be distant from the incoming light rays, since transparency is not a major consideration in the case of the back contact. Metals, on the other hand, are transparent to light rays only when applied in extremely thin layers. They have therefore been used mainly as the back contact in solar cells. The back contact generally has been fabricated from such metals as aluminum, silver, molybdenum, titanium, or nickel.
In efforts to reduce the cost and improve the electrical efficiency of photoconductive devices, several different techniques for the deposition of the various layers comprising such devices have been explored. For example, the deposition of zinc oxide films by a chemical vapor deposition process is disclosed in copending application of Pantham I. Vijayakumar et al., Ser. No. 741,081, filed June 4, 1985, assigned to the same assignee as the present invention. Aluminum and other metals have been sputtered upon the photoconductor to form the back contact in TFS photoconductive devices. However, sputtering involves the use of relatively high temperatures, which may have adverse effects upon photoconductors. Sputtering also results in the deposition of somewhat non-uniform films. In addition, sputtering is slow, difficult to control and involves the use of relatively expensive equipment.
It is accordingly an object of the present invention to provide a photoconductive device having good electrical efficiency at lower cost.
It is another object of this invention to provide such a device which contains a conductive metal film uniformly deposited upon a photoconductor.
Another object of this invention is to provide a method for depositing such conductive metal films in a facile manner.
Other objectives and advantages of the present invention will become apparent from the following detailed description.